Month: February 2011

The Radio Active Workshop kicked off a couple weeks ago in the engineering department of the University of Florida led by yours truly! Jimmy Lin (an aerospace engineering graduate student) set up the group, and together we’re trying to provide an open environment for engineering students (or anyone who’s interested) to meet, share ideas, and get some practical hands-on experience building stuff. We made gave it a radio theme so it meshes nicely with the Gator Amateur Radio Club. I brought a lot of my equipment from home (components, o-scope, tools, etc) and we all started building right away!

This was the first time many students worked on copper boards, as most of them spend their time working with breadboards and/or computer simulations. For a couple it was the first time picking up a soldering iron (how exciting!). My goal is to start everybody off building the same thing (an ultra-simple multi band radio receiver) with a modular design, then turn everyone loose to modify it to their liking. There are a lot of possibilities, from computer control, micro-controller interaction, frequency measurement, stability testing and compensation, audio processing, and of course making it transmit (which should be trivial!). I’m very excited, but still a bit cautious – I think it’s too early to tell whether or not this will be a worthwhile success, or misdirected enthusiasm! I’ll give it my all and see where it goes…

One thing that struck me as a challenge is the difference in levels of experience of the group. We have everything from undergraduate freshmen to experienced graduate students all working on the same project. You can imagine how each of us look at the same circuit differently! I hope that some of the more experienced students can help those less experienced (I fit in that group!) gain some knowledge and come up with some ideas to improve the project.

There are many other projects which would be fun to work on! A cheap and simple frequency counter would be a fun project, especially for the micro-controller gurus out there. Then there’s the enclosure problem – I hope to get a mechanical engineering student to help me out in that department. It would be nice to have a design for an inexpensive HF receiver that can be produced in a small quantity and made available for check out from school radio clubs to let new hams (or those interested in radio) the ability to listen to HF, learn CW, or decode some digital signals! When I got home my wife and I were talking about it and she gave me a hard time for my devices looking so sloppy, specifically commenting on my soldering. Can you believe it? A nursing student ripping on a dental student’s soldering skills — what a funny life I stumbled into =o) Anyhow, I challenged her to make a circuit (switch-controlled LED with filtering capacitor for smooth fade-off) look pretty, and she did! Although it’s not pictured, I got a snapshot of her building it…

Details of the board won’t be published quite yet. I wish to improve it and finalize the design. A PCB would be nice, but I’m very very very hesitant to
go in that direction. PCBs imply “finished” circuits, and I don’t want to give the impression that any circuit I design shouldn’t be tinkered with to try to improve it! We’ll figure that out as it goes. Here are a couple photos of the modules I’m providing as a starting point for the students to make. So far they’ve only made the center board, and next week I imagine we’ll start on (maybe even complete) the rest…

And of course a video – it’s a bit on the long and redundant side, but it clearly demonstrates what we’re working on at the workshop. Again, note that this board is purely for educational purposes, and the amount of exposed copper in the critical sections (antenna/oscillator) obviously needs to be minimized in more finalized designs.

Here’s my most recent schematic:

WARNING WARNING WARNING! This schematic has a couple problems. First and foremost, pins 2 and 3 are *ACTUALLY* pins 1 and 2 (antenna). Pin 3 should be GROUDNED. Also, the series capacitor between the two ICs was replaced by a 22uF capacitor.

I demonstrated earlier this week that I can generate a wide range of frequencies using a tank circuit and the oscillator in a SA612. I added a little circuitry and hooked the receiver to my homemade indoor 20m/40m dipole antenna and vwa la, audio emerges! Endless improvements can be made, but it demonstrates the functionality of this simple circuit.

Yesterday was the notorious “injection lab” where 80 second-year dental students practice injecting on patients (classmates) for the first time. We all performed 5 injections blocking different nerves, some seemed more invasive than others. It was an interesting day I will probably remember. The photo is of my classmate (Steven) giving his first injection to me! Surprisingly it didn’t hurt as much as I expected.

After the event I went to a GARC (Gator Amateur Radio Club) meeting and then to a workshop I’m helping lead in the engineering building. The specific direction of the workshop is still emerging, but it’s in the realm of home-brew radio devices. I took some of the equipment I’ve made and several people seemed interested in potentially working with me on my projects. It would be wonderful to have actual engineers working on these electrical projects!

I got a big bag of fresh, new, copper clad PC board and I now wish I purchased a big pack months ago! Don’t laugh at me, but I was buying 4”x6” sheets of it at Radio Shack for about $5 a pop – ouch! I probably purchased 3 boards in my lifetime, but at that price you can imagine how careful I was not to use them. I soldered minimally to them, and only used them for the most important, established projects. Wake up Scott! If your experimental platform actually inhibits experimentation, there’s something fundamentally wrong. Anyway, I got a stack of the stuff and I no longer hesitate to grab a fresh board and start working. I made some progress today simplifying my ultra-minimalist functional radio project. Here’s what I came up with!

As you can see, it’s running on 9V batteries! The frequency counter has its own 9V battery and a spiffy new hand-me-down case (originally used for a power supply I think, before which it was a watch case!). The IC is a SA602, SA612, or NE602 (all about the same) direct conversion receiver (Gilbert cell mixer).

I now have a small battery powered handheld frequency counter. SWEET! I need to contrive a spectacular case for it. I can’t wait! It’s probably the most impressive thing I’ve ever made with respect to the “cool factor”. Does it look like a bomb? That probably makes it cooler! It just needs a big red on/off switch labeled “MISSILE LAUNCH”, then it’ll be the coolest thing on the planet! … moving on …

This is the receiver component. It’s about as simple as it gets. No antenna or headphone connector is attached, but doing this is trivial! A resonant front-end filter might make it more sensitive, and add some complexity, so comparisons are needed to get a feel for how much better it really is with one attached.

For this board, I added a buffer chip (74HC240) to take the pretty sine wave and turn it into a higher-power square wave…

The quality of the oscillator is reflected in the smoothness of the sine wave (purity?) and its amplitude (indicating high Q?), though more investigation/research is required to fully understand what makes a good oscillator circuit for this chip. My strategy has been to throw components in the air, let them fall randomly, and eventually something happens and the thing starts oscillating…

I’m ecstatic! Finally I built something that worked the first time. Well… on the 3rd attempt! The goal was to develop a minimal-cost, minimal complexity frequency counter suitable for amateur radio. Although I think I can still cut cost by eliminating components and downgrading the microcontroller, I’m happy with my first working prototype.

I haven’t tested it rigorously with anything other than square waves, but I imagine that anything over 1PPV is sufficient (the input is through a bypass capacitor, internally biased right at the trigger threshold). Counting is accomplished by a 74LV8154N (dual 16-bit counter configured as 32-bit) which displays the count as four selectable bytes presented on 8 parallel pins. The heart of the device is an ATMega16 which handles multiplexing of the display and has a continuously-running 16-bit timer which, upon overflowing, triggers a reset of the counter and measurement of the output. Software isn’t perfect (you can see the timing isn’t accurate) but I imagine its inaccuracy can be measured and is a function of frequency such that it can be corrected via software. Here are some photos…

A PCB is DESPERATELY needed. I’ll probably make one soon. Once it’s a PCB, the components are pretty much drop-in and go! No wires! It’ll be a breeze to assemble in 5 minutes. I wonder if it would make a fun kit? It would run on a 9V battery of course, but a calculator-like LCD (rather than LED) display would be ultra-low-current and might make a good counter for field operation (3xAAA batteries would last for months!)

UPDATE: I found out that the ATMega16 donation was from my friend Obulpathi, a fellow Gator Amateur Radio Club member! He also gave me a pair of ATMega32 chips. Thanks Obul!

This is the current state of my receiver. Unlike earlier designs this one uses NO VARIABLE CAPACITORS! This helps because (a) it reduces cost, (b) makes it easier to build for anyone (it’s hard to hunt down identical variable capacitors), and (c) allows it to be totally voltage controlled so microchip or PC control of frequency becomes trivial. Tuning over the entire 40m band is achieved with 3 LEDs reverse biased acting as varactors (wow!). The knobs are potentiometers. The whole circuit runs on 5v.

About Scott

Scott Harden lives in Gainesville, Florida and works at the University of Florida as a biological research scientist studying cellular neurophysiology. Scott has lifelong passion for computer programming and electrical engineering, and in his spare time enjoys building small electrical devices and writing cross-platform open-source software. more →